A well recognized shortcoming of most home heating systems is their relatively low level of efficiency. Furnaces which burn combustible mixtures, particularly hydrocarbon fuels and atmosphere oxygen, require exhaust flues to externally vent the products of combustion to assure that carbon monoxide and other toxic gases do not accumulate in the home to present a health or safety hazard. Studies have indicated that a significant percentage of the heat from home furnaces is lost through the escape of hot flue gases up the chimney. Other home appliances, particularly hot water heaters, which are fueled by combustible mixtures also lose substantial amounts of thermal energy in the venting of hot exhaust gases.
One prior art approach has been to install thermally actuated valves or dampers within the flues which are carefully calibrated to open at a relatively high temperature which is achieved only when the furnace is in actual operation. Such valves open to allow normal aspiration while the furnace is running and partially close to restrict exhaust gas flow when the furnace is off. Even in the off mode, however, flue dampers must remain open a sufficient amount to permit escape of exhaust gas generated by the furnace pilot light, and therefore, provide a thermal leak. Such devices have been only partially successful in the marketplace inasmuch as they could present a safety hazard under some failure modes. Additionally, and more importantly, such devices only operate to block or prevent the escape of heat during periods in which the furnace is not operating. This heat represents a relatively small portion of the total heat loss through the flue during the overall furnace duty cycle of operation.
Other prior art approaches to capturing some of the heat lost up the flue have suggested the application of heat exchangers, positioned within the flue, which circulate a heat absorbing liquid (water) therethrough. The heated water is then either returned directly to the hot water tank of the home as a supplement to normal hot water generation or is used in a hot water radiator to provide supplemental heat. Although such prior art systems are partially successful in capturing some of the otherwise lost heat of the furnace, they have been employed primarily in hot water heating systems as opposed to forced air heating systems and have been relatively complex and expensive, both in installation and maintenance.
A number of prior art devices employed in forced air type heating systems for building have also recognized the advantage of recovering heat from escaping hot flue gases, and have attempted to devise means for transferring a portion of the heat of the exhausted gas to the area intended to be heated. Such heating systems typically include a heating chamber or furnace provided with a warm air delivery duct and a cool air intake or return. A flue pipe for venting the gases and products of combustion is in communication with the heating chamber. The flue pipe, which normally includes a metallic, heat conducting material, passes through the cool air return duct so that the cool air returning to the heating chamber passes over warm surfaces of a short stretch of the flue pipe. The returning cool air is, in effect, slightly preheated prior to entering the heating chamber. In this manner, the temperature of the air within the heating chamber to be heated is slightly increased. Consequently, the energy required to elevate the temperature of the preheated return air to the desired temperature is reduced.
A system such as that described generally in the preceding paragraph is the subject of a recently issued United States Patent which discloses a heat recovery device which is installed within the cool air return duct of a heating system to transfer the ordinarily wasted heat in the exhaust gases flowing through the flue pipe to the cool return air, thereby preheating the latter and increasing the efficiency of the heating system. The details of the device are drawn toward a complex three stage heat transfer structure which is disposed within a heat conductive tubular member which cooperates to transfer heat away from the gases into the cool return air by means of conduction, convection and radiation processes. A first stage deflects a portion of the gases toward and into heat exchanging contact with the sidewalls of the tubular member and in the second stage directs the remaining portion into a gas pervious heat storage trap. The third stage includes a perforated, heat deflecting and radiating cone structure which cooperates with the first and second stages to produce temperature stratification within the tubular member to further increase heat transfer to the cool return air.
Although devices such as that disclosed above are partially effective to transfer heat to the cool return air duct, such systems have a number of shortcomings. The products of combustion being vented in the duct contain poisonous carbon monoxide as well as other toxic gases which, if somehow were able to leak into the fresh air return could conceivably be circulated through the house and present a hazardous condition. Additionally, air, being a relatively good insulator, is not the most effective fluid for application within high capacity heat exchangers. Finally, such devices are often passive inasmuch as they provide no control of the furnace blower and thus only recover heat when the blower is cycled on. When the furnace blower is not on and air is not passing through the cool air return duct, very little heat would be transferred.
U.S. Pat. No. 2,189,748 to Windheim et al represents another approach taken in prior art heat recovery apparatus. In Windheim, a water heater is installed in the flue of a furnace wherein waste heat is captured by heating the water which subsequently supplements the normal hot water system within the home. Such approaches have limited value inasmuch as the supplemental heat to the hot water system is not always present and thus, a constant hot water temperature within the home is difficult to maintain. Additionally, a rupture of a water heating pipe within the flue could cause the water supply to the home to be directly discharged into the furnace with potential catastrophic results.
Still another prior art approach is disclosed in U.S. Pat. No. 4,136,731 to DeBoer. DeBoer discloses a heat transfer system for use in supplementing the operation of the heating/cooling system of a building and its hot water heating system, which includes a heat exchanger in the flue of the furnace as well as a heat exchanger in the fan (furnace) chamber. A first liquid circulation loop couples the heat exchangers for transferring heat from the flue exchanger to the air moved through the fan chamber heat exchanger. A second liquid circulation loop includes the flue exchanger and the building hot water heater for supplementing the heating of water therein. In the summer months during the cooling mode of the system's operation, cold water employed, for example, for lawn sprinkling is passed through the fan chamber heat exchanger for cooling and dehumidifying air circulated in the building. A valve control system is employed to automatically control the flow path of fluid in the system as a function of detected temperatures.
Although the DeBoer device is partially automated and represents an advance in the art, such devices contain many of the shortcomings described herein above and do not address the common situation of a system of appliances having multiple flues or coordinate operation of the heat recovery apparatus with the overall operation of the furnace itself to maximize heat recovery.
Finding a heat recovery device which overcomes the above outlined problems and reduces dependence on hydrocarbon fuels has recently become more urgent in light of the precipitus increase in the cost of such fuels as well as their predicted shortages.